1. Field of the Invention
The present invention relates to a radio communication apparatus and a radio communication system which is effective for transmit power control in TDD (Time Division Duplex) transmission making a communication by time sharing of transmission-reception with the same frequency.
2. Description of the Related Art
There is a CDMA (Code Division Multiple Access) system available as a Multiple Access in which many communication stations mutually communicate by using a specific frequency band allotted. The CDMA is one of spread spectrum communications.
In the spread spectrum communication, a transmitting node multiplies information-modulated data by a so-called spreading code which is a code of fast rate than information data and transmits the spread data by spreading the spreading code into a wide bandwidth, while a receiving node multiplies a received spread data by the same spreading code on the same timing with the transmitting side.
The CDMA is an access method utilizing the spread spectrum communication wherein a different code is allotted to each user and the same frequency band width can be used by each user even between cells.
By the way, when the CDMA is used by a mobile communication, near far problem is arisen. When a plurality of the mobile stations transmit radio waves by the same transmission power to a base station, the radio waves from the station near to the base station are received in a large power as compared to a station far from the base station.
In the CDMA system, since the same frequency band is owned jointly by all users, the high power radio waves from the station near to the base station tend to cause a large interference to the radio waves from the station far from the base station. In order to accommodate effectively many users within the cells, it is necessary that a signal level arriving at the base station is made equal. One of the technological solutions for the near far problem as described above is to control the transmission power.
Further, as a problem particular to the mobile communication, there is a phenomenon called a fading. The fading is a phenomenon, wherein it is well known that the radio waves rarely arrive at the mobile station directly from the base station, but arrive in mutually interfered forms of many waves reflected, deflected and dispersed by surrounding buildings, etc. And therefore amplitude and phase of received signals are made to fluctuate at random.
Also, the changing speed of the received signals is in proportion to the speed of mobile. A transmission power control can follow up the fading of a certain speed.
Illustrated in FIG. 1 is a functional block diagram of the mobile station in TDD radio transmission system for making a conventional open loop type transmission power control. By the open loop type is meant a mode of making a communication wherein the mobile station voluntarily decides the transmission power to the base station.
This communication system comprises a frame assembly device 500 for framing a data to be transmitted, a modulator 501 for modulating the data, an amplifier 502 for amplifying modulated radio waves, a transmitting/receiving changing switch 503 for changing transmit-receive timing of the TDD, an antenna 504 for transmitting and receiving the radio waves, a demodulator 505 for demodulating a received data, a desired wave signal calculation device 506 for calculating a power of a desired wave signal (a desired wave signal at the time of an actual communication) from the demodulated data, a transmission power control device 507 for deciding a transmission power value of the mobile station, and a frame decomposition device 508 for taking out a data by decomposing the frame-composed data.
The action of the TDD radio transmission system as constituted above will be described in details as follows.
First, the radio waves received from the base station are received by the antenna 504 and input to the demodulator 505 via the transmitting/receiving changing switch 503. In the demodulator 505, the data is demodulated by assuming a propagation path and the demodulated data is input to the frame decomposition device 508 and the desired wave signal calculation device 506.
The frame composed data input to the frame decomposition device 508 is decomposed here and necessary components alone are taken out and output as received data.
On the other hand, in the desired wave signal calculation device 506, calculation of a received signal power is made and the calculated received power value is input to the transmission power control device 507.
In the transmission power control device 507, the transmission power value in the base station is memorized and a propagation loss from the base station to the mobile station is calculated from the transmission power value and the desired wave signal received power value. The transmission power value is represented by the value added with a target level in the base station (the level in which the base station can correctly receive radio waves) and the propagation loss. This transmission power value is calculated by the following expression EQU PMS=PTG+(PBS-RMS) [dBm] (1)
Note that PMS[dBm] is the transmission power value of the mobile station, PTG[dBm] is the target level in the base station, PBS[dBm] is the transmission power of the base station and RMS[dBm] is the desired wave signal received power. (PBS-RMS) is the propagation loss.
The radio waves modulated by the modulator 501 is amplified by the amplifier 502 into the transmission power value which is decided by the above described expression (1) and transmitted from the antenna 504 via the transmitting/receiving changing switch 503. Thus, the transmission power control is effected as above.
However, in the conventional radio transmission system, the received power of the desired wave signal can not be accurately measured if a signal to noise power ratio (SNR) or a signal to interference power ratio (SIR) is low in the mobile station. When the received power value of the desired wave signal is erroneously measured, there are cases where the transmission power is suddenly increased. Also in the case that a cellular telephone is adapted, the moment the mobile station moves into the shady side of buildings, a level of the received signal is suddenly increased. At this moment, there are frequent occasions when transmission is suddenly made with a big power and ,as a result, a big interference is caused to the radio waves of other users mobile stations.